1. Field of the Invention
The present invention relates to a printing head and an ink jet printing apparatus and more particularly to the ejection opening arrangement in a printing head used for bidirectional printing with scanning of the printing head in two directions.
2. Description of the Related Art
For ink jet printing apparatuses such as inkjet printers, speed increase in color printing is one of the major subjects. As a technique for increasing printing speed, it is a general practice to increase the drive frequency to the printing head, to implement bidirectional printing or the like besides increasing the printing head length. Bidirectional printing is effective means as a total system in respect of cost, because of decentralization of the energy required to obtain a given throughput in terms of time. However, in bidirectional printing, the ejecting order of color inks is possibly different between forward and backward scans; thus a fundamental problem, in which a band-like color irregularity having a width of the scanning area occurs, is involved.
As a printing head structure for solving the above problem, Japanese Patent Laid-Open No. H1-208143 (1989) describes a structure that the ejection opening arrays for respective colors of ink are arranged in a sub-scanning direction orthogonal to the scanning direction of the printing head. Also, Japanese Patent Laid-Open No. S58-179653 (1983) describes a structure provided with ejection openings for forward printing and ejection openings for backward printing. In this document, the ejection openings or heads to be used are switched over in between forward and backward scans so that the respective colors of ink are ejected in the same order between the forward and backward scans. The printing head is structured by a combination of printing head components for ejecting respective color inks. Furthermore, Japanese Patent Laid-Open No. S58-215352 (1983) discloses a structure that the printing head is structured with a plurality of head groups to eject different colors of ink wherein the plurality of printing head groups are arranged deviated alternately in the conveying direction of a printing medium. This can increase the arrangement pitch of ejection openings in the color printing head, with respect to a desired image density.
However, in the structure as described in Japanese Patent Laid-Open No. H1-208143, the printing head has an increased length in the sub-scanning direction with a result that size increase is incurred for the apparatus. On the other hand, in the structure a plurality of printing heads are combined as described in Japanese Patent Laid-Open Nos. S58-179653 and S58-215352, the apparatus problematically increases in size in the scanning direction because of increased width of the printing head with respect to the scanning direction. Such size increase of the printing head in the scanning direction leads to increasing scanning time, thus not being desirable in the viewpoint of high-speed printing.
For the above problems, the printing head disclosed in Japanese Patent Laid-Open No. 2001-171119 arranges a plurality of ejection opening arrays for respective color inks in one body thus achieving a compact head structure. This printing head has two ejection opening arrays for each color of ink, which are separately used in forward and backward scans so that the ejecting order thereof can be identical in the forward and backward scans. The head structure is made compact by providing, in the central portion of the head, each ink supply passage commonly to the same color of ejection opening arrays for use in forward and backward printings.
In the meanwhile, concerning the structure to cope with forward and backward printings by means of a symmetric arrangement of ejection opening arrays for respective ink colors as disclosed in Japanese Patent Laid-Open Nos. S58-179653, S58-215352 and 2001-171119, there is further known a printing head structure that the symmetrically-arranged ejection opening arrays include an array of ejection openings made different in ejection opening size or in ejection volume. This allows for ejecting a small ink droplet and a large ink droplet to perform printing by ejecting the small ink droplet in the high-resolution print mode and by ejecting the large ink droplet in the high-speed print mode. However, in this printing head structure, when the large and small sized (large and small ejection volumes of) ejection openings are separately used in accordance with the print mode, there possibly encounters a problem of density unevenness due to deviation of landing positions of ejected inks resulting from the arrangement positions of the ejection opening arrays.
FIGS. 7A and 7B illustrate this problem. In the printing head shown in FIG. 7A, large-sized ejection opening arrays 21a, 25b and small-ejection opening arrays 21b, 25a are provided, for example for cyan (C1, C2) ink. A set of a large-ejection opening array 21a and a small-ejection opening array 21b and a set of a small-ejection opening array 25a and a large-ejection opening array 25b are arranged symmetric with each other. In this printing head structure, printing is made by using only the small-ejection opening arrays 21b, 25a as to cyan ink in the high resolution mode or so. In this case, there encounters a possible case that the ejection opening array 25a is wholly in a positional relationship inclining upward or downward relative to the ejection opening array 21b with respect to the scanning direction (in the left-right directions in the figure), due to the error in mounting the printing head or so. In the figure, illustrated is an example showing inclining θupward. Particularly in the printing head structured such that ejection opening arrays for respective colors are integrated as described in Japanese Patent Laid-Open No. 2001-171119, inclination relationship as above arises where there are assembly tolerances of the printing head or mounting errors on the printer body. In such a case, the cyan ink ejected from the ejection opening array 25a lands at a position wholly deviated upward or downward from the normal position, relative to the landing position of the cyan ink ejected from the ejection opening array 21b. As a result, overlaps and gaps arise between the dots thus formed as shown in FIG. 7B, which causes density unevenness such as stripes in a printed image.
Such problems arise likewise in the mode using the large-ejection opening arrays. However, the large-sized dot, even if it deviates from the normal position, causes less change of density on the whole because of its comparatively greater area as represented on the printing medium. Namely, where forming a dot smaller in size, density change becomes more conspicuous due to gaps and overlaps of dots, which in turn appears in the form of density unevenness. In addition, the ink, whose spacing is greater in its symmetric arrangement, appears more conspicuous with respect to density. In the example of FIGS. 7A and 7B, overlaps or gaps of dots are more conspicuous as to ink C1, C2 than as to ink M1, M2. More specifically, provided that a dot-to-dot distance is “d”as to the same color ink, deviation is related to d sin θ, thus meaning that overlaps or gaps of dots increase commensurate with the distance “d”.
As described so far, where using a printing head having sets of large-sized (large ejection volume of) ejection openings and small-sized (small ejection volume of) ejection openings for ink of the same color wherein two sets thereof are arranged symmetric with each other, the problem of density unevenness may be possibly encountered due to the manufacture errors of the printing head, depending on a certain print mode or so.